Can't connect to Azure SQL with error 18456

I'm trying to grant access to my Azure SQL database (on an elastic pool) to an external developer using SQL Authentication. In the database (not master) I ran the following commands: create user [UserName] with password='Password', default_schema=[dbo] alter role db_owner add member [UserName] He can't access the database, he gets the error: `Login failed for user 'UserName'. (.Net SqlClient Data Provider) Error Number: 18456 Severity: 14 State: 1 Line Number: 65536` Both our IPs have been whitelisted, but only I can login to the database with those credentials. Any idea where things are going wrong?